In pulping of wood chips, it has been recognized that the thickness dimension of the wood chips plays an important role in the quality of the pulping process. During pulping, a digester receives chips and, through the use of chemicals, pressure and elevated temperatures, the wood is broken down into its constituents which include lignin and cellulose. The cellulose or wood fibers are then processed for making the pulp product. The thickness (or smallest dimension) of the chip is critical (as opposed to its length) since the thickness dimension determines the effectiveness of the alkaline digesting chemicals in penetrating to the center of the chip. As is recognized by those skilled in the art, in producing a uniform high yield pulp, providing a correctly sized and composed chip flow is extremely important.
Oversized and overthick chips are not properly broken down in the digester and can result in a reduced pulp yield due to the subsequent removal of these particles during the pulping process. Undersized chips typically include pins and fines, with pins comprising chips which are smaller than a desired chip size range, and fines even smaller particles such as sawdust or small bark particles. The undersized chips should also be removed from the chip flow which is fed to the digester, since undersized material can be overcooked in the digester resulting in a weakening of the overall pulp.
Thus, it is necessary to provide a flow of chips to the digester which is acceptable from a standpoint of having low levels of overthick chips and low levels of undersized chips. While complete removal of oversized and undersized chips is not necessary, and in fact is generally not practically or economically possible, the acceptable flow to the digester should contain overthick chips below a certain percentage and undersized chips below a certain percentage of the overall flow. The particular percentages which are deemed allowable in an acceptable flow (to the digester) can vary from pulping mill to pulping mill.
Chip screening systems are well-known. Many screening systems in use today are described in an article by E. Christensen appearing in the May 1976 TAPPI Journal, Vol. 59, No. 5. A gyratory screen is one type of screening device which provides high particle separation efficiency for given screen sizes. Gyratory screens have less of a tendency to upend and remove elongated particles such as pin chips, and there is less tendency to plug the screen openings with particles close to the screen opening size. Gyratory screens agitate the wood chips, causing the smaller particles to migrate downwardly toward the screen surface for removal. In addition, gyratory screens have less tendency to abrade and break chips into smaller pieces. Thus, gyratory screens effectively remove fines and retain pins, in separating the pins and fines from the wood chip flow.
Another typical screening device, as disclosed in the Christensen article is known as the disk screen. A disk screen includes a number of parallel rows of shafts upon which spaced rotating disks are mounted such that the disks on one shaft are axially spaced between the disks on an adjacent shaft. The spacing determines the size of chip that will fall through and those that stay atop and pass over disk screen. When the flow is large, and deep, a smaller proportion of the chips will have access to the spacing or slots between the disks. Thus, the flow rate (and the depth of the flow) also plays a role in determining the fraction of chips which pass through the screen. The rotation of the disks aids in orienting and to some extent urging the chips through the slots. Varying the rotational speed can therefore also affect the proportion of chips passing through the slots, though generally to a less extent than the spacing and flow rate. As described in the Christensen article, the disk screen will separate "overs", or in other words oversized and overthick chips, from the remainder of the flow, since the " overs" will generally not pass through the spacing between disks of adjacent shafts of the disk screen.
In one system described by Christensen, it is suggested to first pass an incoming chip flow over a disk screen to remove the "overs" fraction. The fraction which passes through the disk screen (i.e., between the disks of adjacent shafts) will contain the chips which are acceptably sized as well as pins, fines, sawdust, etc. The "overs" will be processed further to reduce their size to within a predetermined acceptable range of sizes. This system/method is the most commonly practiced today, and is known as a "Primary Thickness Control," since the primary thickness controlling unit is the first stage in the process.
Another chip sizing process is disclosed in U.S. Pat. No. 4,376,042 to Brown, in which an incoming flow of chips is divided into three fractions utilizing a gyratory screen. One fractional output flow includes an acceptable flow of chips. A second fraction includes acceptable chips as well as the oversized and overthick chips. The second fraction is directed to a disk screen which separates the overthick and oversized chips from the acceptable chips. The acceptable chips from the second fraction as well as the acceptable chips from the first fraction are then fed to the digester. The third fraction includes the undersized chips which are then removed from the system, and may be transported for example to a fuel bin.
The process described in the Brown patent was implemented in 1986 at the Weyerhauser Longview, Washington mill. The Weyerhauser process has proven successful in providing a "sustained high performance" chip thickness and chip uniformity system as well as providing a low maintenance operating system. The Brown/Weyerhauser process is viewed as a high performance chip thickness and uniformity system and currently ten systems utilizing this process are in use or under construction. While the relatively new Weyerhauser process is a significant advance in the industry it is important to note that systems which utilize a primary disk thickness screening process exceed 140 in the industry.
While the use of a disk screen as a primary thickness screen (in which oversized and overthick chips are separated from an incoming flow) has gained widespread acceptance it is constantly a goal to provide improved chip screening systems which can provide acceptable chip flows to digesters as economically as possible. Moreover, it is important that any such improvements be compatible with existing systems, such that existing systems may be retrofitted, thereby avoiding the tremendous capital outlay required for completely new systems.